1. Field of the Invention
The present invention relates to a painted metal sheet, on which a printed design full of variety is given with a sublimation dye, for use as a multi-colored signboard, a decorative interior sheet, a decorative surfacing sheet, a door panel for an elevator or an outer panel for electric home appliances or furniture, etc.
2. Description of Related Art
Painted metal sheets printed with colorful designs have been manufactured so far by offset, silk, photogravure or transfer-printing. In a conventional transfer-printing method, a sublimation dye is applied to a topcoat or clear paint formed on a painted metal sheet by a proper printing method such as offset, silk, photogravure or transfer. The top clear layer is then impregnated with the sublimation dye by heat treatment. For instance, JP 51-24313A discloses a method wherein a transfer film is heated in contact with a paint layer of thermosetting synthetic resin. JP 54-104907A discloses a method wherein a paint layer printed with a sublimation dye is formed on a metal sheet, a top paint layer is formed on the printed layer, and then the top layer is impregnated with the sublimation dye from the inner side by heat treatment. JP 7-31931A discloses a method wherein a pre-coated metal sheet, which has a primer paint layer and a colored top paint layer, is impregnated with a sublimation dye. JP 7-102733A discloses a method wherein an opaque resin layer formed on a metal sheet is impregnated with a sublimable coloring agent.
In any case, an objective design appears by penetration of the sublimation dye into the top paint layer. However, a conventional clear paint layer is inferior of anti-scratching property (scratch resistance), wear-resistance and slippage-proof (slippage resistance) property, and scratches formed on its surface are apparently distinguished, although it is smooth, glossy and vivid. In this consequence, the printed sheet is not applicable to such a part as a flooring sheet or a table counter, which is used under abrasive conditions.
Anti-scratching property and wear-resistance of a paint layer can be improved by addition of an inorganic filler. For instance, JP 48-66640A proposes a powdery paint improved in anti-scratching property and wear-resistance by addition of glass fibers at a ratio of 5-70 wt. %. JP 51-8128A proposes a paint, which contains glass flakes having thickness less than 3 μm and a size passing a sieve of 150 meshes, for a precoated steel sheet improved in anti-scratching property and wear-resistance. JP 8-183926A proposes a painted metal sheet coated with an acrylic resin paint improved in anti-scratching property and wear-resistance by addition of an inorganic filler at a ratio of 5-60 parts by weight based on 100 parts by weight of a solid vehicle in a paint. However, these paint layers are poor of transparency to well intensify a colored design using as a background metallic luster of a substrate metal or a color tone of an undercoat paint layer and also inferior of slippage-proof, although they have good anti-scratching property and wear-resistance. Anti-scratching property and wear-resistance of a paint layer can be improved by addition of an inorganic filler. For instance, JP 48-66640A proposed a powdery paint improved in anti-scratching property and wear-resistance by addition of glass fibers at a ratio of 5-70 wt. %. JP 51-8128A proposed a paint, which contains glass flakes having thickness less than 3 μm and a size passing a sieve of 150 meshes, for a precoated steel sheet improved in anti-scratching property and wear-resistance. JP 8-183926A proposed a painted metal sheet coated with an acrylic resin paint improved in anti-scratching property and wear-resistance by addition of an inorganic filler at a ratio of 5-60 parts by weight based on 100 parts by weight of a solid vehicle in a paint. However, these paint layers are poor of transparency to well intensify a colored design using as a background metallic luster of a substrate metal or a color tone of an undercoat paint layer and also inferior of slippage-proof, although they are good of anti-scratching property and wear-resistance.
A paint layer is also improved in anti-scratching property and wear-resistance by irradiation with an electron beam, as noted in an electron beam-curing acrylic paint (as disclosed in JP 55-5422B, JP 56-8070B, JP 1-229622A and JP 2-242863A). Since a paint layer irradiated with an electron beam has hardness of 9H or harder by a pencil hardness test, it has good wear-resistance, anti-scratching property and anti-fouling property. However, such an electron beam-curing paint layer is poor of plasticity and relatively expensive, and also needs a special electron beam irradiator for curing the paint layer, resulting in increase of a manufacturing cost. There is also the disadvantage that a paint layer cured with electron beam irradiation is poor of wear-resistance, compared with a thermosetting resin layer.
By the way, vinyl chloride tiles, vinyl chloride panels, etc., which are commonly used as organic flooring materials are difficult to give a multi-colored design with a sublimation dye due to poor dimensional stability and poor heat-resistance. Decorative flooring material, which uses metallic luster of a substrate metal sheet as a background for a multi-colored design, is scarcely offered to the market. Most table counters are made of wood, but multi-colored goods with metallic appearance are scarcely offered to the market.
Multi-colored decorative signboards have been also manufactured so far by a short-lot process wherein a decorative film is individually stuck to a metal sheet or a painted sheet instead of using a sublimation dye. However, such a decorative signboard can not be used for a long term exceeding a half-year, since the laminated decorative film is easily peeled off. It is also difficult to increase hardness of the decorative film for improvement of anti-scratching property, accounting for lamination of the decorative film at a final stage of the manufacturing process. In addition, external appearance of the signboard is significantly influenced by texture of the decorative film, so that it is impossible to allot color with metallic or ceramic impression.
Coloring concentration of a pattern printed with a sublimation dye is limited to a narrow range due to poor masking ability of the sublimation dye. When a heat is applied to a transfer film during a transfer-printing step, a sublimation dye is often excessively transferred even to an undercoat paint layer or reversely transferred to the transfer film. Such unfavorable transfer of the sublimation dye causes a printed pattern to lack sharpness especially in case of printing characters or the like.
A decorative design is realized by impregnation of a top paint layer with a sublimation dye in any of conventional design-printing methods. However, such a sublimation dye is a dispersion-type or oily type having a small polarity, and is easily degenerated by plasticizers or organic chemicals, and also decomposed by ultraviolet irradiation resulting in discoloration or fading. Due to these unfavorable properties of the sublimation dye, the decorative design is hardly kept in a stable colored state under conditions exposed to open air for a long time. Discoloration or fading caused by ultraviolet irradiation can be inhibited by addition of a proper ultraviolet-absorbing agent to a paint at a ratio of 0.5-3 wt. % on the basis of a non-volatile component in the paint.
A precoated steel sheet as a substrate for transfer-printing is manufactured by baking an applied paint at 200-240° C. (as a highest temperature of a substrate sheet) for 1-2 minutes, while a paint layer is impregnated with a sublimation dye at 160-190° C. for 1-4 minutes. That is, the ultraviolet-absorbing agent added to the paint is exposed to a high-temperature atmosphere at least two times until a final stage of a printed metal sheet-manufacturing process. A commonly used ultraviolet-absorbing agent such as benzophenone or benzotriazole is quantitatively decreased in the paint layer due to poor resistance to heat and sublimation. The weight loss of the ultraviolet-absorbing agent puts harmful influences on discoloration or fading of the decorative design, but also causes deformation of the paint layer to a yellowish rugged surface. Such yellowish appearance is apparently distinguished, when paint-baking as well as transfer-printing are performed at a higher temperature.
Weight loss of the ultraviolet-absorbing agent is suppressed by addition of a thermally-stable and well-soluble benzotriazole or triazine compound at a ratio of 6-18 wt. % on the basis of a non-volatile component in a paint, as disclosed in JP 9-206678A. Addition of such a benzotriazole or triazine compound is effective for many kinds of sublimation dyes, but discoloration or fading of some sublimation dyes can not be suppressed to a level necessary for outdoor application. Although discoloration or fading may be suppressed by using a high-grade sublimation dye for good light resistance, change of the sublimation dye is not a practical idea accounting constraint on aptitude, color tone, etc. of the sublimation dye in correspondence with a type of a printer used for outputting a decorative image. For instance, if only one color ink is poor of light-resistance among basic 4 colors (cyanic, magenta, yellow and black), a printed sheet can not be used for outdoor application.